The present invention relates to a magneto-optical recording and reproducing apparatus in which the recording, reproduction and erasure of information are made by applying a laser beam and a magnetic field to a magnetic recording layer film provided on a disk-like substrate.
A recording operation in a generally known magneto-optical recording and reproducing apparatus includes converging or focusing a laser beam onto a magnetic recording layer formed on a disk substrate so that a minute region of the magnetic recording layer irradiated with the laser beam is heated up to the vicinity of a Curie temperature. An external magnetic field from a magnetic field generating means is applied to the heated minute region to turn the magnetization of the minute region to a direction of the external magnetic field, thereby effecting the recording of a signal. Upon erasure, on the other hand, the recorded signal is erased by carrying out a similar operation to the recording operation while making the direction of the external magnetic field reverse to that at the time of recording.
As the magnetic field generating means, a permanent magnet or an electromagnetic coil is generally used. The strength of the magnetic field necessary for the recording or erasure of a signal is in the order of 300 to 500 Oe.
When the permanent magnet is used as the magnetic field generating means, it is relatively easy to obtain the above-mentioned magnetic field strength. However, the inversion of the magnetic field cannot be rapidly made since it must be carried out by changing the direction of the permanent magnet by use of a mechanical means. Therefore, the apparatus using the permanent magnet has a drawback that it cannot be used for applications in which the recording and erasure are performed at a high speed (for example, over writable recording).
On the other hand, when the electromagnetic coil is used as the magnetic field generating means, the inversion of the magnetic field can be easily made. In order that the strong magnetic field of 300 to 500 Oe is applied to a minute region on the disk substrate by the electromagnetic coil, it is necessary to keep the electromagnetic coil near to the magnetic recording layer as much as possible. However, the disk rotates with a runout in its thickness direction. (The runout in the thickness direction will be hereinafter referred to generally as axial runout.) Accordingly, it is impossible to allow the electromagnetic coil to come near the magnetic recording layer with a distance smaller than the maximum amount of axial runout. Also, a distance between the electromagnetic coil and the magnetic recording layer varies during rotation of the disk because of the axial runout. Therefore, the magnetic field strength at the magnetic recording layer changes and hence a stable signal recording or erasure operation cannot be expected. Particularly, in the case where the disk substrate is made of plastics, the abovementioned problem is severe since the amount of axial runout of the disk substrate may come up to .+-.0.5-.+-.1 mm.
A proposal for solving this problem has been disclosed by, for example, JP-A-60-209944. According to the disclosed proposal, a laser beam used for the operation of recording, reproduction and erasure of information transmits through a magnetic recording layer and the beam transmitted through the magnetic recording layer is used as a light for detection of the amount of deviation in position of an electromagnetic coil. The positional deviation of the electromagnetic coil is detected on the basis of the transmitted beam by a system exclusively used for the purpose of error detection including a convex lens, photo-electric converter, etc. The detected signal representative of the positional deviation is supplied to a driving means to correct the deviation in position of the electromagnetic coil with respect to the magnetic recording layer.
In the above-mentioned prior art technique, that portion of the laser beam used for the recording, reproduction and erasure of information which transmits through the magnetic recording layer of the disk is used for detecting the deviation in position of the electromagnetic coil in the disk thickness direction with respect to the magnetic recording layer. Therefore, this prior art technique has a problem that it cannot be applied to a disk having a reflective magnetic recording layer which is impermeable to a laser beam. Further, the system exclusively used for the purpose of error detection including the convex lens, photo-electric converter, etc. is required as a means for detecting the positional deviation of the electromagnetic coil. Therefore, there is a problem that the construction is complicated.